US President Barack Obama cancelled a meeting with foul-mouthed Philippine leader Rodrigo Duterte on Tuesday at a regional summit after being branded a “son of a whore” who would wallow like a pig.

The pair were due to meet in the Lao capital of Vientiane at a gathering organised by the Association of Southeast Asian Nations, an annual event meant to foster harmony but which often highlights regional rows.

This year’s edition was launched with a spectacular fallout between the United States and the Philippines, longtime allies that have seen relations plunge under a barrage of insults from Duterte since he came to office on June 30.

Obama’s aides announced that his planned meeting with Duterte on Tuesday afternoon had been called off following a fresh tirade by the Filipino leader the previous day.Shortly before flying to Vientiane, Duterte warned he would not be lectured by Obama over a war on crime in the Philippines that has claimed more than 2,400 lives in just over two months.

“You must be respectful. Do not just throw away questions and statements. Son of a whore, I will curse you in that forum,” Duterte told reporters when asked about his message for Obama.

– Porcine threat –Duterte, who has quickly earned a global reputation for his acid-tongue, then used typically colourful language to describe their planned meeting if rights issues came up.

“We will be wallowing in the mud like pigs if you do that to me,” he said.

Duterte had previously also branded the US ambassador to Manila a “gay son of a whore”, and sought to taint the reputation of Pope Francis’s mother in similar fashion.

Duterte was elected to office in a landslide this year after pledging to kill 100,000 people in an unprecedented war on crime.

When faced with criticisms over an apparent spate of extrajudicial killings in his crime war by the United Nations, he responded with what has become familiar abuse.

“Maybe we’ll just have to decide to separate from the United Nations. If you are that disrespectful, son of a whore, then I will just leave you,” he said last month.

But faced with the Obama backlash on Tuesday, Duterte offered a rare moment of contrition, albeit qualified.

“While the immediate cause was my strong comments to certain press questions that elicited concern and distress, we also regret that it came across as a personal attack on the US President,” he said in a statement.

– Crucial time –The setback in US-Philippine relations comes at a crucial time in the region, with China seeking to cement control over the contested South China Sea.

The Philippines, Vietnam, Malaysia, Brunei and Taiwan also have competing claims to the strategically vital waters, but have watched China expand its presence by building artificial islands in key locations.

An international tribunal ruled in July that China’s claims to the waters — through which $5 trillion in global shipping trade passes — had no legal basis.

The verdict was widely seen as a sweeping victory for the Philippines, which filed the suit under the previous administration of Benigno Aquino.

But China has vowed to ignore the ruling.

And Duterte has sought to heal relations with China rather than inflame them by pressing the tribunal’s ruling.

Under Aquino, the Philippines had forged closer military ties with the United States to deal with the China threat. But Duterte has cast doubt on that strategy.

Obama’s aides had previously said he wanted to discuss the South China Sea issue with Duterte in Laos.

Nevertheless, the South China Sea issue is expected to once again be discussed at the three days of meetings hosted by ASEAN, which will be attended by Chinese Premier Li Keqiang.

The gathering will see the 10 ASEAN members meet by themselves, then with leaders from the US, China Japan, South Korea and China.

Other leaders to come for an East Asia summit on Thursday include those from Australia, India and New Zealand.

Laos is the final Asian visit of Obama’s eight-year presidency, during which he has sought to refocus American military, political and economic resources on the region.It is also the first visit by a sitting US president to Laos, which the United States secretly carpet bombed for nearly a decade in the Vietnam War, killing tens of thousands of people.

Obama is expected to on Wednesday announce greater help in clearing the bombs.